(1) Field of the Invention
This invention relates to a heating apparatus which makes use of light (hereinafter referred to as "radiant heating apparatus"), and more specifically to a radiant heating apparatus equipped as its heat source with lamps such as, for example, halogen lamps so as to irradiate light onto an item to be heated (hereinafter called the "object") and heat it up and adapted for annealing ion-implanted semiconductor wafers, forming oxide films on semiconductor wafers, inducing a chemical reaction or crystal growth on the surface of an object or like purposes.
(2) Description of the Prior Art
Heating apparatus of various types have heretofore been employed for the above-described purposes. Among such known heating apparatus, radiant heating apparatus are advantageous for the heat treatment of silicon semiconductor wafers for the following reasons:
(a) The heating temperature can be rapidly raised or lowered since the heat capacity of each lamp per se is extremely small;
(b) The control of heating temperature can be readily effected by controlling the electricity to be supplied to each lamp;
(c) Objects will be free from contamination since they are heated by light radiated from each lamp without being brought into contact with the heat source;
(d) Energy consumption is smaller because a full-radiation-state operation of each lamp may be achieved shortly after its actuation and the lamp enjoys high energy efficiency;
(e) The apparatus is smaller in size and less costly, compared with other heating furnaces such as resistive furnaces and high-frequency furnaces.
FIG. 1 is a simplified schematic fragmentary view of an example of conventional radiant heating apparatus. In the drawing, numeral 1 indicates plane mirrors which individually define a number of grooves 1a extending in parallel to one another. A tubular halogen lamp 2 is disposed in each of these grooves 1a, thereby making up plane light-source units 10. A plurality of such plane light-source units 10 are arranged up and down or side by side with a space 7 therebetween so as to construct a radiant heating apparatus. The space 7, defined between the units 10, serves as a heating space in which objects are placed for their heat treatment.
Such a radiant heating apparatus may be used as is for heating such objects that are not deleteriously affected when heated in the ambient atmosphere. It however cannot be used as is, where the heat treatment has to be carried out under specific atmospheric conditions. In the heat treatment of a semiconductor wafer for instance, in order to effect desired heat treatment uniformly throughout the wafer, it is required to heat the wafer with uniform temperature distribution to a prescribed temperature under a constant atmospheric condition where a specific gas composition, pressure and the like are maintained unvaried. For example, in order to form a protective film of silicon nitride on the surface of a silicon semiconductor wafer, it is necessary to raise the temperature of the wafer evenly while causing a gas mixture, which consists of a carrier gas of argon and hydrazine (N.sub.2 H.sub.4) and silane (SiH.sub.4) gases mixed with the carrier gas, to flow through the heating chamber in which the wafer is placed. However, it is indispensable to use a container, which isolates the heating chamber from the ambient atmosphere, in order to pass a specific gas through the heating chamber. Moreover, the gas flow in the container must be uniform at the position where the wafer is placed. Since the plane light-source units, which are adapted to heat the wafer placed in the container through the transparent wall of the container, are designed in such a way that the temperature distribution of the wafer exposed to the light from the plane light-source units is maintained uniform in the course of its heating, an uneven gas flow will lead to a non-uniform cooling effect and thus to uneven temperature distribution of the wafer. As a result, the composition and thickness of a protective film to be formed will become uneven.